Gold and gold-coated substrates are typically bonded to itself or other materials utilizing thermocompression bonding, eutectic bonding, and epoxy bonding. Thermocompression bonding requires the use of moderate temperatures (>250° C.) and pressure (>1 MPa) to bond gold-to-gold. Eutectic bonding uses a metal amalgam to bond gold to another surface (i.e. Au/Sn). Epoxy bonding uses an organic epoxy based adhesive with various additives (adhesion promoter, filler, conductive materials) to bond gold to various substances (i.e. TRA-BOND 868-6N3, ABLEBOND 789-3).
Bonding a variety of materials (e.g., glasses, glass ceramics, single crystals, and polycrystalline materials) to a gold or gold-coated surface with a bonding agent that is durable, versatile, well-adhered, and optically transparent is a very difficult task. Gold and gold-coated substrates are difficult to bond due to the chemical inertness of the gold surface and limited means to form bonds to the surface. See, for example, Cotton, F. A., Wilkinson, G. Advanced Inorganic Chemistry 5th Edition, 1988 John Wiley & Sons, New York pg. 939. Numerous organic agents (epoxy-and organo-silanes) have been utilized to bond a variety of materials with optical transparency; however, this results in bonded materials that are temperature limited due to the adhesive working temperatures (typically <200° C.) and may significantly outgas under vacuum conditions. One common problem with organic bonding agents is a bond that can be hydrated, swell, and delaminate upon exposure to water and/or other chemical agents. For example, U.S. Pat. No. 7,053,521 states that a gold surface is very difficult to bond to and discloses a method for laminating gold surfaces by adding a thiol containing organosilane to a bisphenol-A epoxy adhesive for better adhesion. Few chemical species can bond to gold, and even those that do, for example alkanethiols, only form uniform films to gold surfaces through dative bonds, but do not have the requisite strength and stability to function as an adhesive. Traditional organic bonding materials are suitable for use on both optically and non-optically flat surfaces; however, they often have limited shelf-life, and/or limited pot life, and may require mixing prior to dispensing due to the reactivity of the components of the adhesive. For example, TRA-BOND 868-6N3™, an epoxy for bonding of gold and various metals, has a pot life of 15 minutes, shelf-life of 3 months at −40° C. storage, and operating temperature up to 115° C.
Inorganic bonding agents have been used to bond glasses and glass ceramics; however, this usually requires a precision optical finish in order to produce defect free bonds with a thickness of ˜2 microns. Inorganic bonding methods that can be performed at low temperatures (<350° C.) typically require optically flat surfaces to produce transparent bonds and specific surface chemistries. For example, U.S. Pat. No. 6,284,085, which discloses a hydroxide containing bonding solution for bonding various materials together, is limited to materials having oxide surfaces.
Thus, there is a need in the art for a gold bonding procedure without the constraints of the prior art gold bonding materials and methods.